American English 

"Rain check" means "to postpone". So if, say, a baseball game is cancelled due to heavy rain, patrons are given a compensatory ticket to attend the next game. This means there is a certainty that the event will happen; it’s just a matter of time.

British English 

The phrase has another meaning: "Let's wait until the time comes and see if it's still a good idea". This is because it rains so frequently in Britain that it is quite normal to wait until the last minute before deciding whether or not to embark on an activity.

Source: Edited from the english@work e-newsletter by 938LIVE. Subscribe to the english@work newsletter.